For almost 20 years, workers and visiting school children at a Grand Canyon museummay have been unknowingly exposed to radiation from three buckets of uranium sitting next to a taxidermy exhibit, according to allegations from a National Park Service safety director.

The whistleblower says officials learned about the buckets last year and tried to hide the revelation, according to the Arizona Republic newspaper. Earlier this month, Elston “Swede” Stephenson emailed all park staff and brought the matter to the attention of the head of the interior department, which oversees the park service, and the agency’s internal watchdog.

The interior department told CNN that a recent survey showed radiation levels at “background” levels, which it said are always in the environment because uranium naturally occurs in the rocks of the Grand Canyon.

“There is no current risk to the public or park employees,” the department said.